For quite some time, various methods have been proposed to easily build a custom kernel from the -ARCH one. Search the Wiki for a few examples. All these were intelligent ideas, but suffered from [some drawbacks] and were not officially supported by the devs.

On the contrary, the method described in this article should be more solid and safe, and builds on the kernel26 official package.

Install ABS

Getting the Ingredients

First of all, you need a clean kernel to start your customization from. In this article I will assume that you use the official kernel26 -ARCH kernel. So create a folder you want to work in and get the kernel package files from ABS (after syncing):

Modifying the PKGBUILD

Changing pkgname

As you see, there is a commented line for building a kernel with a different name. All you need to do here is to uncomment that line, change the suffix '-custom' to your needs, and comment the standard line. For instance, your file could become:
Template:File

Note: This assumes that you do not need to recompile kernel26-headers, -manpages or -docs. If you do, change all three strings accordingly.

Now, all the variables of your package will be changed according to the new name. For instance, after installing the package the modules will be located at Template:Filename.

Changing build()

You probably need a custom .config file for your kernel. You can uncomment one of the possibilities shown in the build() function of the PKGBUILD, e.g.:
Template:File

If you have already a kernel config file, I suggest to uncomment one interactive config tool, such as nconfig, and load your config from there. This avoids problems with kernel naming I have met with other methods.

Note: If you uncomment return 1, you can change to the kernel source directory after makepkg finishes extraction and then make nconfig. This lets you configure the kernel over multiple sessions. When you're ready to compile, copy the .config file over top of either config or config.x86_64 (depending on your architecture), comment return 1 and use makepkg -i.

Changing the package_kernel26() function

Now, you have to write a custom function to tell your system how to install the package. This is most easily done by changing the name of the package_kernel26() function to package_kernel26-test(), and adapting the instructions to your needs. If you have no particular needs, your package_kernel26-test() should look like this:
Template:File

Compiling with multiple CPUs

To tell the compiler to use all CPUs during compile use the -j<number> make flag. The number should be n+1 where n is the number of your CPUs.

Compiling

Now you can proceed to compile you kernel by the usual command
Template:Codeline
If you have chosen an inteactive program for configuring the kernel parameters (like menuconfig), you need to be there during the compilation.

Note: A kernel needs some time to be compiled. 1h is not unusual.

Installing

After the makepkg, you can have a look at the kernel26.install file. You will see that some variables have changed. Now, you only have to install the package as usual with pacman (or equivalent program):
Template:Codeline

Boot Loader

Now, the folders and files for your custom kernel have been created, e.g. /boot/kernel26-test-img . To test your kernel, update your bootloader (/boot/grub/menu.lst for GRUB) and add new entries ('default' and 'fallback') for your custom kernel. That way, you can have both the stock kernel and the custom one in parallel.

Traditional

This method involves manually downloading a source tarball, and building in your home directory as normal user. Once configured, two compilation/installation methods are offered; the traditional manual method as well as makepkg/pacman.